1. Field of the Invention
The present invention relates to a die for pressmolding optical elements with high precision at low cost and also preferably provided with improved durability and a method for preparing the same.
2. Description of the Prior Art
When manufacturing an optical element such as an aspherical glass lens with high precision, hitherto, the glass was directly ground and machined by a machine with extremely high performance. In such conventional method, it took a very long time to manufacture each lens, and so it was not suited to mass production.
Recently, as a method of manufacturing optical elements with high precision in quantities and at low cost, the press-molding method has been noticed, and variously devised. To mold optical elements with high precision by press-molding, durable dies with high precision are needed. In other words, the press-molding die for optical elements must be stable both thermally and chemically at high temperature, resistant to oxidation, inert to materials of the optical element, and excellent in mechanical strength so that the die shape may not be deformed during press-molding. To the contrary, the die must be excellent in machinability so as to be machined precisely and easily.
Conventionally, as the die material satisfying to a certain extent the above mentioned necessary conditions, silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) as disclosed in Japanese Laid-open Patent No. 52-45613 have been used. Besides, titanium carbide (TiC) mixed with metals as disclosed in Japanese Laid-open Patent No. 59-121126 may be considered also as the die material. The dies made of such materials are high in strength, but these materials are likely to react with lead (Pb) or alkali components contained in the optical elements, and so the dies may be adhered to the optical elements after several times of press-moldings, and become unusable. Besides, since these dies are very hard, or excellent in mechanical strength, the diamond tool used for machining the dies is worn out quickly, which makes it difficult to machine the dies precisely.
More recently, it has come to be possible to mass-produce optical elements by press-molding, using the press-molding die as disclosed in Japanese Patent Publication No. 62-28091 as shown in FIG. 1. This die is composed of a base material 11 which is precisely machined to an inverted shape of an optical element, and a surface protective layer 12 which is formed on the base material 11 so as not to spoil the surface shape of the base material. The die base material 11 is made of a cemented carbide mainly composed of WC which is excellent in heat resistance and mechanical strength, so as to maintain the strength of the die. The press surface of the base material 11 is precisely machined to the inverted shape of the optical element. The base material 11 is coated with a platinum alloy thin firm as the surface protective layer 12 which is thermally and chemically stable at high temperature, resistant to oxidation, inert to the optical element material, and excellent in mechanical strength so as not to deform the die shape during press-molding. It is intended to press-mold the optical elements repeatedly. It is intended to press-mold the optical elements repeatedly. In this die, however, since it took a long time to precisely machine the base material 11 made of the cemented carbide mainly composed of WC, so that the die manufacturing cost was very high, and the optical elements could not be mass-produced at low cost.
A new die as shown in FIG. 2 is proposed hence in order to greatly shorten the die machining time as disclosed in Japanese Patent Application No. 60-140824. This die is composed of a base material 21 roughly machined to an inverted shape of an optical element, an intermediate layer 22 precisely machined on the base material 21, and a surface protective layer 23 formed so as not to spoil the surface shape of the intermediate 22. The die base material 21 is made of tungsten carbide, zirconia, cermet, silicon nitride or silicon carbide excellent in heat resistance and mechanical strength in order to maintain the strength of the die. The press surface of the base material 21 is roughly machined to the inverted shape of the optical element. A gold, copper, nickel of platinum-gold, platinum-copper, or platinum-nickel alloy thin film with the platinum content of 60 wt. % or less excellent in cutting processability and grinding processability is formed on the base material 21 as the intermediate layer 22. The intermediate layer 22 is precisely machined to the inverted shape of the optical element, and then coated with the surface protective layer 23 which is a platinum alloy thin film that is stable thermally and chemically at high temperature, resistant to oxidation, inert to optical element materials, and excellent in mechanical strength. Therefore, the optical elements can be press-molded repeatedly, while the die machining time is shortened remarkably. Therefore, by using this die, optical elements with higher precision can be mass-produced at lower cost. In this die, however, when the press-molding operation is repeated many time, the intermediate layer 22 is separated from the base material 21 because the adhesion between the base material 21 and the intermediate layer 22 is not so strong, so that a large number of optical elements cannot be press-molded by one die. Therefore, in order to realize manufacturing of optical elements with higher precision at lower cost, it is essentially required to further reduce the die machining time. In a case of using a material excellent in grinding machinability and cutting machinability as the intermediate layer, the adhesion strength between the base material and the intermediate layer is not sufficient for durability of the die, that is, stress concentration in the intermediate layer and repeating of expansion-shrikage caused by the molding temperature cycle make the intermediate layer, adhered to the base material, tend to be peeled off and be cracked. Therefore, in order to realize manufacturing of optical elements with higher precision at furthermore lower cost, it is also essentially required to enhance the adhesion strength between the base material and the intermediate layer thereby to improve the durability of the die.